A recurrent and often bitter controversy in the study of intelligence is the issue of how heredity and environment contribute to intelligence. Arthur Jensen, a leading figure in the debate, has contributed both data and argument to the “nature” view. Which of the following statements represents an important assumption, rather than an inference or an observation, in Jensen’s argument? Circle the letter of the best answer, and explain why it is the best answer and why each other answer is not as good.

a) Identical twins have identical genetic endowments.

b) Identical twins should have IQs that are more similar than the IQs of ordinary siblings.

c) The correlation between IQs of twins reared together is .89.

d) Differences between the correlations of IQs for twins reared together versus those of twins reared apart indicate that environment has only a weak effect on intelligence.

e) The environments of twins reared together versus those of twins reared apart are very different.